SVT with aberrant conduction due to the Wolff-Parkinson-White syndrome

The most important distinction is whether the rhythm is ventricular (VT) or supraventricular (SVT with aberrancy), as this will significantly influence how you manage the patient. SVTs usually respond well to AV-nodal blocking drugs, whereas patients with VT may suffer precipitous haemodynamic deterioration if erroneously administered an AV-nodal blocking agent.

Unfortunately, the electrocardiographic differentiation of VT from SVT with aberrancy is not always possible.

There are several electrocardiographic features that increase the likelihood of VT:

Absence of typical RBBB or LBBB morphology

Extreme axis deviation (“northwest axis”)

Very broad complexes (>160ms)

AV dissociation (P and QRS complexes at different rates)

Capture beats — occur when the sinoatrial node transiently ‘captures’ the ventricles, in the midst of AV dissociation, to produce a QRS complex of normal duration.

Fusion beats — occur when a sinus and ventricular beat coincides to produce a hybrid complex.

Brugada’s sign – The distance from the onset of the QRS complex to the nadir of the S-wave is > 100ms

Josephson’s sign – Notching near the nadir of the S-wave

RSR’ complexes with a taller left rabbit ear. This is the most specific finding in favour of VT. This is in contrast to RBBB, where the right rabbit ear is taller.

Examples of these ECG features are shown below:

Capture beats

Fusion beats - the first of the narrower complexes is a fusion beat (the next two are capture beats)

Positive concordance in VT

Negative concordance in VT

Brugada’s sign (red callipers) and Josephson’s sign (blue arrow)

Taller left rabbit ear in VT

Taller right rabbit ear in RBBB

The likelihood of VT is also increased with:

Age > 35 (positive predictive value of 85%)

Structural heart disease

Ischaemic heart disease

Previous MI

Congestive heart failure

Cardiomyopathy

Family history of sudden cardiac death (suggesting conditions such as HOCM, congenital long QT syndrome, Brugada syndrome or arrhythmogenic right ventricular dysplasia that are associated with episodes of VT)

The likelihood of SVT with aberrancy is increased if:

Previous ECGs show a bundle branch block pattern with identical morphology to the broad complex tachycardia.